The present invention relates to optical switching, and particularly to an apparatus and method for optical switch alignment.
Communications networks are increasingly becoming all optical networks, incorporating optical switching. Optical switches are typically fabricated using micro electro-mechanical systems (MEMS) technology. A recently developed optical switch of this type is described in xe2x80x9cPerformance of a 576xc3x97576 Optical Cross Connectxe2x80x9d by H Laor, A Richards, E Fontenot, Proceedings of the National Fibre Optic Engineers Conference Sep. 28, 1999, Chicago, USAxe2x80x9d; which is incorporated herein by reference. Such switches may be used to switch wave division multiplex (WDM) signals as a group, or the WDM signals may be demultiplexed outside the switch and switched individually as channels, or as groups of channels as desired. MEMS switches use moveable mirrors to redirect and align optical paths between switching modules within the switch. The switching paths must be correctly aligned to complete an optical signal or channel connection across the switch.
There is a need to set up optical paths through the switch before there is an optical signal or channel present. That is the optical path must be set up and verified before transmission of optical channels to prevent misconnection through misalignment of the moveable mirrors in the switch, and hence to prevent the introduction of errors into the optical signal by the switching function of the network.
FIG. 1 shows a schematic diagram of a MEMS based optical switch incorporating a prior art optical path alignment arrangement. The optical switch comprises a number of port cards 1 coupled to modules of a MEMS core switch 2 comprising focusing lenses, fixed and moveable mirrors as described below, and in more detail in the above reference for example. Each port card 1 corresponds to a pair of modules in the switch 2 and connects optical fibres (for example 3a and 4a) to modules of the MEMS core switch 2, which switches incoming fibres (3a, 3b . . . ) to desired outgoing fibres (4a, 4b . . . ). A typical core network switch has 156 pairs of modules.
The optical signal wavelength band is typically determined by standards, for example on or around 1500 nm for core network transmission. The prior art arrangement uses light of a different wavelength injected into the optical path to enable correct alignment of the mirrors in the MEMS core switch 2. Light from a local light source 5 within each port card 1 is injected into the optical path 3 using a wavelength selective coupler 6. Another wavelength selective coupler 6 on the other side of the MEMS core 2 of each card 1 is used to filter off the light injected which is then detected by a receiver or detector 7, and used to indicate that the MEMS mirrors are correctly aligned. A major disadvantage of this arrangement however is the addition of two wavelength selective couplers 6 and a local light source for each port card 1 of the switch. This adds significantly to the per port cost of the switch fabric.
It is an object of the present invention to provide an improved apparatus and method for optical switch alignment.
In a first aspect the present invention comprises an optical switching apparatus comprising:
an optical switch having a number of optical paths for switching optical channels;
a demultiplexer coupled to said switch;
a broadband light source coupled to said demultiplexer which distributes said light in said channels to enable correct alignment of said optical paths.
Preferably said apparatus further comprises receiver means coupled to said switch and arranged to indicate correct alignment of said optical paths by receiving said light.
Preferably said receiver means comprises local receivers for each said channel.
Preferably the light source is an E-LED.
Alternatively the light source is amplifier spontaneous emissions.
Preferably the light source is modulated.
Preferably said modulation is amplitude modulation.
Preferably the switch is an MEMS switch.
In a second aspect the present invention comprises an optical switching apparatus comprising:
an optical switch having a number of optical paths for switching optical channels;
a demultiplexer coupled to said switch;
a modulated broadband light source coupled to said demultiplexer which distributes said light in said channels to enable correct alignment of said optical paths.
In a third aspect the present invention comprises an optical switching apparatus for enabling correct alignment of optical paths in an optical switch, said apparatus comprising:
a demultiplexer coupled to said switch;
an in-band light source coupled to said demultiplexer which distributes said light in demultiplexed optical channels to enable correct alignment of said optical paths.
In a further aspect the invention comprises a method of aligning optical paths through an optical switch for switching a number of optical channels, the method comprising:
generating a broadband light source in a multiplexed signal comprising said channels;
demultiplexing said signal;
receiving said light in the demultiplexed channels to indicate correct alignment of said optical paths.
In a yet further aspect the present invention comprises a computer program on a machine readable medium, said computer program being capable of performing a method of controlling an optical switching apparatus to align optical paths through an optical switch for switching a number of optical channels, the method comprising:
introducing a broadband light source into a multiplexed signal comprising said channels;
demultiplexing said signal;
receiving said light in the demultiplexed channels to indicate correct alignment of said optical paths.